The present invention relates to an electrophotographic photoconductor and more particularly to an electrophotographic selenium-tellurium photoconductor.
As a method of increasing the photosensitivity of an electrophotographic selenium photoconductor, a method of adding tellurium to the selenium photoconductor is known. However, the characteristics of selenium may be considerably impaired depending upon the amount of tellurium added or the manner of its addition, so that the selenium photoconductor may not be sufficiently sensitized and accordingly cannot be used in practice.
In order to improve that point, an electrophotographic photoconductor plate comprising a photosensitive layer comprising a selenium-tellurium alloy and halogen is proposed in Japanese Laid-open Patent Application Ser. No. 50-142036, in which the concentration of tellurium in the photosensitive layer is uniform in the direction parallel to the surface of the photoconductor, while the concentration of tellurium gradually increases in the direction normal to the surface of the photoconductor. More specifically, the concentration of tellurium near the surface of the photoconductor is in the range of 5 to 20 wt. %, while the concentration of tellurium is not more than 5 wt. % near the base plate.
Furthermore, a selenium electrophotographic photoconductor has been proposed, in which the concentration of tellurium in the photosensitive layer increases in the direction normal to the surface of the photoconductor and the concentration of tellurium is higher near the surface of the photoconductor. However, when such a selenium electrophotographic photoconductor is used in practice in an electrophotographic copying machine, black lines appear in the copy due to uneven abrasion of the photosensitive layer.
Generally, the photosensitivity of a photoconductor is proportional to the product of (i) the number of charge carriers produced in the photoconductor by the light projected thereto and (ii) the drift mobilities of the charge carriers. In the photoconductor comprising a selenium-tellurium alloy, the number of charge carriers to be produced by the light projected thereto depends upon the concentration of tellurium contained in the 1.about.3.mu. surface layer of the photoconductor, while the drift mobilities of charge carriers also depend upon the concentration of tellurium in the inner layer below the surface layer. If the concentration of tellurium contained in the 1.about.3.mu. surface layer of the photoconductor is constant, the photosensitivity of the selenium-tellurium alloy photoconductor depends upon the drift mobilities of charge carriers. The drift mobilities of charge carriers in the inner selenium-tellurium layer are minimum when the concentration of tellurium therein is about 4 wt. % and increase in a parabolic shape (if plotted) outside the above-mentioned minimum range of about 4 wt. %.
Therefore, such a range of concentration of tellurium as minimizes the mobilities of charge carriers in the selenium-tellurium alloy layer has to be avoided.
Practically, in the electrophotographic photoconductor comprising an electroconductive base and a photosensitive layer formed thereon, which comprises a selenium-tellurium alloy, it is preferable that the concentration of tellurium in the photosensitive layer be 5 wt. % or more, more preferably 8 wt. % or more, and that the concentration be substantially uniform in the direction normal to the surface of the photoconductor, from the view point of the photosensitivity of the photoconductor.